1. Field of the Invention
This invention relates to a semiconductor laser apparatus with the use of vertical cavity surface emitting laser diode (hereinafter, referred to as VCSEL), and in particular, a semiconductor laser apparatus and fabrication method thereof, the semiconductor laser apparatus being in use as a light source for optical information processing and optical communication or the light source of a data storage memory apparatus that stores data by means of laser beam.
2. Description of the Related Art
With increased data communication capacity of these years, there is a demand for a semiconductor laser apparatus having a higher speed and higher reliability, in which the semiconductor laser diodes are formed into an array. The laser diode is mounted on a package in accordance with the standard TO-CAN packaging in general usage, yet it is known that the die bonding of the chip greatly influences the operation life and reliability of the chip.
In general, when the chip such as VCSEL or the like is die-bonded, a die attachment is utilized for bonding. Therefore, it is important to select a die attach material that a thermal expansion coefficient is close to that of the laser chip, the bonding strength is excellent, and thermal conductivity is high. For this reason, the bonding method is employed with a metal alloy-based die attach material such as Au—Sn solder, Sn—Pb solder, or the like, which is excellent in heat release.
For instance, Japanese Patent Application Publication No. 2004-22608 (hereinafter, referred to as Document 1) describes an anchor structure in which multiple mountain-valley patterns are formed on a boundary between an Au—Sn alloy and a solder layer that includes Sn subsequent to bonding in order to strengthen the solder bonding.
Japanese Patent Application Publication No. 2003-200289 (hereinafter, referred to as Document 2) describes that when two members are bonded to form an Au—Sn alloy, the mass fraction of Sn is configured equal to or less than 13 percent by mass in the composition of the Au—Sn alloy in the bonding portion, subsequent to bonding.
Japanese Patent Application Publication No. 2000-261085 (hereinafter, referred to as Document 3) describes a laser diode apparatus in which the laser diode is soldered to a submount and the submount is soldered to the stem. The solder layers are respectively formed in the laser diode and the submount before the solder joint.
Japanese Patent Application Publication No. 6-37403 (hereinafter, referred to as Document 4) describes a structure in which the tin film for mounting the semiconductor laser diode on the submount is surrounded by the gold film having same thickness so that the gold-tin alloy-based solder may not be extruded to the back end surface of the semiconductor laser at the time of die-bonding the semiconductor laser diode. The laser beam emitted from the back end surface is not diverged by the solder, thereby reducing the deterioration with age of the monitoring current.
Japanese Patent Application Publication No. 2004-55692 (hereinafter, referred to as Document 5) describes a structure that includes the submount, the semiconductor laser diode, and the alloy layer disposed between the submount and the semiconductor laser diode. The alloy layer is composed of a portion of the electrode of the semiconductor laser diode and the solder layer provided on the submount in advance, and the Sn content included in the alloy layer is configured equal to or less than 26 percent by mass. It is possible to suppress Sn to condense in the laser beam emitting region during operation, by reducing the Sn content. This prevents the laser oscillation from stopping.
The package having the chip such as VCSEL or the like mounted thereon, sometimes operates under high temperatures. For instance, in a selectively oxidized VCSEL, a distortion is generated due to the difference in the thermal expansion coefficient between the current confinement region having the oxidation region and the active region provided close thereto. This adversely affects the operation characteristics of the optical output and the like in some cases. Therefore, it is necessary to suppress the heat generated in VCSEL as much as possible. This is why the metal solder material of Au—Sn or the like having an excellent heat release characteristic is employed for the die attach of the chip. However, the metal solder material has a higher bonding temperature than that of the plastic die attach material of a sliver paste or the like, and this high temperature thermally affects the inside of the VCSEL. Consequently, there is the probability that the reliability of the device will be degraded. In contrast, if the bonding temperature between the metal solder materials is lowered, the bonding strength between the metal solder material is reduced. The chip is, in some cases, peeled from the die attach or mounter in a subsequent bonding process or the like.
Document 1, on the other hand, describes that a hard and weak Au—Sn compound is difficult to be formed because the solder layer that includes Sn and the Au—Sn alloy layer are well blended together. However, it is difficult to control the mountain-valley patterns on the boundary with high repeatability. If the partially weak Au—Sn compound is formed, there is a problem that the chip will be peeled from the weak portion.
In addition, Document 2 proposes the object of preventing remelting of solder in a former bonding when the laser chip is bonded to the heat sink. However, if the Sn content is too small, it is easy to form an Au-rich layer that is hard and weak. This causes drawbacks that the chip is peeled from the mount at the time of driving or the operation life is shortened because of the thermal stress generated during operation. As described heretofore, there are several problems that should be solved to obtain the semiconductor laser apparatus in which the bonding strength between the metal solder material and the chip is kept with the high reliability.
With a semiconductor laser apparatus or a package thereof in accordance with the present invention, the above-described problems or drawbacks can be solved by including Au and at least one of constituent materials of the die attach material, except Au, in the material for a lower electrode.